The aflatoxins are a family of carcinogenic mycotoxins of polyketide origin. Aflatoxin B1 is the most potent liver carcinogen known for the laboratory rat. These toxins occur widely in many parts of the world as food contaminants and are an important environmental health hazard. The biosynthesis of this family of toxins is to be investigated using specific 2H/13C-, 3H/14C- and 13C/180-labeled specimens of averufin, nidurufin, 6-deoxyversicolorin A and potential ortho-carboxybenzophenone and dienone intermediates in the bisfuran and xanthone rearrangement steps. These experiments will establish unequivocally the fate of isotopically-labeled centers by Ft-nmr or degradation. Mechanistic and stereochemical information will be sought in these experiments and biomimetic syntheses will be attempted to explain the in vitro chemistry of certain reactive systems.